In a long optical fiber communications link, chromatic dispersion of the fiber media causes portions of an intensity modulated signal to arrive at the receiver at slightly different times. This temporal pulse distortion leads to intersymbol interference and effectively limits the usable bandwidth of the fiber. The dispersion experienced through a given fiber is proportional to both the wavelength carried and the length of the fiber.
A typical single mode fiber will exhibit a dispersion of about 17 picoseconds per nanometer per kilometer (ps/nm-km) at a wavelength of 1545 nanometers. The dispersion changes with a positive slope of about 0.09 ps/nm-km per nm as the carrier wavelength is varied. Thus, at wavelength 1560 nm, the same typical fiber would have a dispersion of about 18.4 ps/nm-km.
One solution to the high dispersion characteristics of typical fibers at currently used wavelengths involves the use of dispersion shifted fiber. Dispersion shifted fiber is constructed so as to have nearly zero dispersion at a wavelength of about 1550 nm. However, dispersion shifted fiber still has a delay slope, which results in nonzero dispersion values at all other wavelengths. Thus, dispersion shifted fiber offers only limited relief in a multi-wavelength environment.
A substantial amount of existing fiber is of the non-dispersion shifted variety. Engineers have sought ways to expand the usefulness of these installed fibers rather than replace them. One common technique is to incorporate dispersion compensation into the regenerative devices that are spaced along the fiber path. For this purpose, a special dispersion compensating fiber (DCF) has been developed. One commonly available form of dispersion compensating fiber has a dispersion of −80 ps/nm-km at a wavelength of 1545 nm and a negative slope of −0.15 ps/nm-km. Specific lengths of DCF can be inserted periodically into a fiber system to reduce the dispersion effects.
Another way of performing dispersion compensation involves the use of a Bragg grating or dispersion compensating grating. A dispersion compensating grating is a length of fiber in which the index of refraction of the fiber varies at selected spacings along the length of the fiber. This causes selective reflection of wavelengths that are congruent to the grating spacing. If the spacing between these variations is constant along the length of the grating, then the grating will reflect a narrow range of wavelengths whereas all other wavelengths will simply pass through the grating. A Bragg grating may be used to exhibit a dispersion slope by gradually varying the spacing along the length of the grating. The result is referred to as a “chirped” grating. One form of dispersion compensating grating may be made by gradually decreasing the spacings between the refractive index variations. This causes shorter wavelengths to travel further down the fiber grating before being reflected, in other words, before encountering spacings comparable to the wavelength. In this arrangement, shorter wavelengths will experience longer travel times resulting in a negative dispersion slope. By the appropriate selection of length and range of spacings, a dispersion compensating grating can be designed to compensate for a given dispersion characteristic. A dispersion compensating grating of only 10 cm in length can replace tens of kilometers of dispersion compensating fiber.
Because of the manner in which it is fabricated, a typical dispersion compensating grating exhibits a delay versus wavelength response that is not perfectly smooth. A dispersion compensation grating introduces variations in delay as a function of wavelength that may be described as “ripple” upon the dispersion characteristic curve. This ripple can lend sufficient phase variation (dispersion) over a narrow range of wavelengths so as to impair significantly the quality of a modulated optical channel. In situations where the average compensation of the dispersion compensating grating may appear adequate for use in a given optical path, the localized delay ripple characteristics of the dispersion compensating grating may render it unsuitable for use at some wavelengths and modulation rates.